The present invention relates to the art of earth boring and, more particularly, to a rotary rock bit with improved means for sealing the bearings from materials in the borehole. The present invention is especially adapted for use in that type of rotary rock bit popularly known as a three-cone bit; however, its use is not restricted thereto, and the present invention can be used in other types of rotary rock bits.
A three-cone rotary rock bit is adapted to be connected as the lowest member of a rotary drill string. As the drill string is rotated, the bit disintegrates the earth formations to form an earth borehole. The three-cone rotary rock bit includes three individual arms that extend angularly downward from the main body of the bit. The lower end of each arm is shaped to form a spindle or bearing pin. A cone cutter is mounted upon each bearing pin and adapted to rotate thereon. Individual bearing systems promote rotation of the cone cutters. The bearing systems have traditionally been roller bearings, ball bearings, friction bearings, or a combination of the aforementioned bearings. The cone cutters include cutting structure on their outer surfaces that serve to disintegrate the formations as the bit is rotated. Various forms of seals have been provided between the cone cutter and the arms upon which they are mounted; however, the prior art seals have in many instances proven to be less than satisfactory.
The rotary rock bit must operate under very severe conditions, and the size and geometry of the bit is restricted by the operating characteristics. At the same time, the economics of petroleum production demand a longer lifetime and improved performance from the bit. In attempting to provide an improved bit, new and improved materials have been developed for the cutting structure of the cones. They have provided a longer useful lifetime for the cones. This has resulted in the bearing system of the bit being generally the first to fail during the drilling operation. Consequently, a need exists for improved bearing systems to extend the useful lifetime of the bit. In attempting to improve the bearing system, various seal systems have been provided to maintain the bearing area free of harmful materials. In attempting to provide an improved bearing system incorporating an improved sealing system, great care should be taken that the overall capacity of the bearing system is not reduced.
When a sealed bearing rotary rock bit is lowered into a well bore, the environmental pressure surrounding the bit increases at the rate of approximately 1/2 pound per square inch for each foot of depth. This means that at a depth of 10,000 feet the hydrostatic pressure on the outside of the bit could be 5,000 psi or more because of the weight of the drilling fluid in the well bore above the bit. In order for a lubrication system to function properly at the elevated down hole pressures, some means must be provided to equalize the internal pressure of the lubricant in the lubrication system with the hydrostatic pressure of the drilling fluid in the well bore. Pressure equalization has previously been accomplished by closing one end of the lubricant reservoir with a flexible membrane and venting the portion of the reservoir on the outer side of the membrane to a port on the surface of the bit. This system has only been partially successful because the flexible membrane tends to rupture under large pressure differentials and the pressure differential across the seal tends to push the seal element into a position where it is damaged by other elements of the bit.
A wide variety of other environmental conditions also affect the performance of the pressure equalizing system. For example, the temperature will rise as the well bore penetrates deeper into the earth and temperatures of up to 325.degree. F are presently being encountered. Deep wells now being drilled are expected to result in environmental temperatures of up to 400.degree. F and the drilling of steam wells results in environmental temperatures as high as 550.degree. F. The elevated temperatures have an adverse effect on the lubricant, the structural elements of the lubrication system, the structural elements of the pressure equalizing system and a substantial increase in the pressure of the lubricant within the lubricant reservoir is generally encountered.